"It is under development right now and should be deployable in the coming months," press secretary Geoff Morrell was quoted saying in a report by Defense News.

U.S. Pentagon officials said they had asked Congress in August to redirect $52 million in funding to the project in order to speed up production of the massive bomb.

After winning congressional approval, the Pentagon said this week that it had awarded Boeing's McDonnell Douglas a $51.9 million contract to "enable B-2 aircraft" to carry the bomb.

"The threats have been developing over the years," a Pentagon spokesman was quoted saying to U.S. media. "There are, without getting into any intelligence, there are countries that have used technologies to go further underground and to take those facilities and make them hardened. This is not a new phenomenon, but it is a growing one."

He said the first of the bombs would be ready by the middle of 2010.

While the United States possess similar bombs in its arsenal, the "bunker buster" is said to take that capability of penetrating hardened facilities to new levels.

Pentagon officials refused to clarify whether the bomb's development was in response to Iran's controversial nuclear arms program.

Last month Defense Secretary Robert Gates said an attack against Iran would only "buy time," delaying Tehran's dispute nuclear program by about three years.